1. Field of the Invention
This invention relates to a new method of making super-conducting Josephson junctions. More particularly, this invention relates to a method of planarizing Josephson junction devices which substantially improves the yield as well as the circuit density. More generally, this invention relates to a method of planarizing any metal conductor line in an integrated circuit. 2. Description of the Prior Art
Numerous published articles have described the problems that arise as a result of attempting to fabricate Josephson tunnel junction devices. Some of these problems relate to thermal cycling and others relate to the actual process of building up the layers which comprise a Josephson tunnel junction device. When layers of a Josephson tunnel junction device are built up, they are usually made by the process of vacuum deposition. It is well known that the process of vacuum deposition forms a substantially uniform layer over the area being coated. It is also well known that Josephson junction devices comprise a base electrode and a counter electrode with a Josephson tunnel junction therebetween. In the preferred embodiment of making Josephson junction devices, the base electrode is first made on a substrate as a raised discrete area. Preferably this base electrode is a very small area to achieve high packing density on the substrate. As vacuum deposited layers are built up on top of this base electrode, they replicate the contour and generate what is called a step. Heretofore, layers built up over the base electrode have been made much thicker than is desirable for optimizing the electrical properties in order to achieve a smoothing effect. However, this smoothing effect is not to be confused with planarization.
Prior art process steps employed in the manufacture of semiconductor devices are to some extent capable of accomplishing smoothing effects or planarization. For example, when applying a photoresist to an irregular surface, it is possible to use a thicker photoresist which will achieve some smoothing effect. By applying subsequent layers of such photoresist, the smoothing effect can finally achieve a surface which is substantially planarized. As a further example, it has been possible to coat a single layer of thick photoresist two micrometers thick over a surface having irregular raised surfaces two tenths of one micrometer in height. The photoresist was cured in a known way and the result achieved was a surface planarized within 200 .ANG.. Another material which has been used, when applicable, to planarize a layer is a liquid solution containing polyamic acid which is cured to form a polyimide layer. The use of photoresist, polyamic and other organic base liquids to form insulation layers is not applicable to Josephson junction devices because of the extreme temperatures encountered in operation. None of the aforementioned known examples of planarization are related to planarizing when forming layers by vacuum deposition. The only known examples of prior art planarization are achieved through having a material which will flow in a semi-liquid state.
It would be highly desirable to provide a method of planarization which could be carried out while vacuum depositing layers of a Josephson junction device. If such planarization could be achieved, it would be expected that the yields and circuit density of the Josephson junction integrated circuit could be substantially improved. Planarization of the layer permits better line width control because surface irregularities degrade line width. Electrical properties are more controllable when uniformly thick layers can be obtained.